Here is REST handler.

@RestResource(urlMapping='/UpdateContact/*')
global with sharing class REST_ParticipantHandler {
  
    @HttpPost
    global static void updateParticipant() {
        
        //rest context variable initialization.
        RestRequest req = RestContext.request;
        RestResponse res = RestContext.response;
        //utill class returns fieldmap from the custom setings
        Map<String,String> FieldMap=util.getFieldMap();
        Map<String,String> FieldTypeMap=util.getFieldTypeMap();
        Map<String, Object> jsonBodyMap = (Map<String, Object>)JSON.deserializeUntyped(req.requestBody.toString());
        Blob body = req.requestBody;
        String bodyString = body.toString();
        Wrapclass deserializedinput = (Wrapclass)JSON.deserialize(bodyString, Wrapclass.class);
        Map<String, Object> jsonBodyMapLcase = new Map<String, Object>();
        string qFields=String.join(FieldMap.values(), ', ');
        string uNum=''; 
        String sf_jsonResponse = '';
        //converting jSON body map keys into lowercase to handle casesensitity issues.
        for( string s1:jsonBodyMap.keySet()){
            jsonBodyMapLcase.put(s1.toLowerCase(),String.valueOf((String)jsonBodyMap.get(s1)));
            system.debug(LoggingLevel.error,'jsonBodyMap#'+s1+'='+String.valueOf((String)jsonBodyMap.get(s1)));
        }
        
        //getting the userNum from Jsonbody to instantiate corresponding participant record.
        if(!string.IsBlank((String)jsonBodyMap.get('userNum'))){
            uNum = !string.IsBlank((String)jsonBodyMap.get('userNum'))?String.valueOf((String)jsonBodyMap.get('userNum')):'';
        }
        
        system.debug(LoggingLevel.error,'uNum#'+uNum);
        system.debug(LoggingLevel.error,'jsonBodyMap#'+jsonBodyMap);
        // No userNum parameter was found; return status 400
        if(uNum == null) {
            res.statusCode = 400;
            sf_jsonResponse = '{"response": {"status": "Failure", "message": "MissingRequiredQueryParameter userNum"}}';
            res.responseBody = blob.valueOf(sf_jsonResponse);
            return;
        }
        //Dynamic query to query the object dynamically with all the fields.
        string query ='SELECT ' + qFields
                       + ' FROM contact'
                       + ' WHERE user_num__c = \'' + String.escapeSingleQuotes(uNum) + '\''
                       + ' LIMIT 1';
        system.debug(LoggingLevel.error,'query#'+query);
        List<Contact> parts=Database.query(query);
        // No Participants with matching userNum
        if( parts.size()<=0) {
            res.statusCode = 400;
            sf_jsonResponse = '{"response": {"status": "Failure", "message": "No matching Participant record:'+uNum+' ' +'found with this userNum"}}';
            res.responseBody = blob.valueOf(sf_jsonResponse);
            return;
        }
        //participant Instance
        contact p=parts.get(0);
        
        for(String pfield:FieldMap.keySet()){
            
            if(!string.IsBlank((String)jsonBodyMapLcase.get(pfield))){
                
                if('Text'==FieldTypeMap.get(pfield))    {
                    p.put(FieldMap.get(pfield),String.valueOf((String)jsonBodyMapLcase.get(pfield)));}
                
                else if('Currency'==FieldTypeMap.get(pfield)){
                    p.put(FieldMap.get(pfield),Decimal.valueOf((String)jsonBodyMapLcase.get(pfield)));}
                
                else if('Number'==FieldTypeMap.get(pfield))  {
                    p.put(FieldMap.get(pfield),Integer.valueOf((String)jsonBodyMapLcase.get(pfield)));}
                
                else if('Date'==FieldTypeMap.get(pfield))    {
                    system.debug(LoggingLevel.error,'Field=' + FieldMap.get(pfield));
                    //Data coming as a string value with '-' seprator and salesfore wont accept the string to date field. needs to convert string to salesforce date formate.
                    string sDate=string.valueOf((String)jsonBodyMapLcase.get(pfield));
                    system.debug(LoggingLevel.error,'sDate**=' + sDate);
                    //date d =util.getDateSfFormate(sdate);
                    String[] myDateOnly = sDate.split(' ');
                    String[] strDate = myDateOnly[0].split('-');
                    Integer myIntYear = integer.valueOf(strDate[0]);
                    Integer myIntMonth = integer.valueOf(strDate[1]);
                    Integer myIntDate = integer.valueOf(strDate[2]);
                    Date dt= Date.newInstance(myIntYear, myIntMonth, myIntDate);
                    system.debug(LoggingLevel.error,'dt**=' + dt);
                    p.put(FieldMap.get(pfield),dt);}
                else if('Checkbox'==FieldTypeMap.get(pfield)){
                    if(String.valueOf((String)jsonBodyMapLcase.get(pfield))=='1'){
                        p.put(FieldMap.get(pfield),True);} 
                    else {
                        p.put(FieldMap.get(pfield),False);}
                }
            }
            else{
                p.put(FieldMap.get(pfield),null);}
        }
        system.debug(LoggingLevel.error,'updated Participant=' + p);
            
        try {
            res.statusCode = 200;
            update p;
            sf_jsonResponse = '{"response": {"status": "Sucess", "message": "Participant record:'+uNum +' ' +'is updated in Salesforce"}}';
            res.responseBody = blob.valueOf(sf_jsonResponse);
            return;
        } catch ( Exception ex ) {
            res.statusCode = 500;
            sf_jsonResponse = '{"response": {"status": "Failure", "message": "Participant record:'+uNum +' '+ ex + '"}}';
            res.responseBody = blob.valueOf(sf_jsonResponse);
            return;
        }
        
    }
     global class Wrapclass {
        public String status;
        public String responseBody;
        
    }
    
}

Here is my Test class..

@isTest(seeAllData=false)
Public class Test_RESTParticipantHandler {

    public static testMethod void testdoPost() {
        
        System.RestContext.request = new RestRequest();
        System.RestContext.response = new RestResponse();
        
        Account a = new Account(Name = 'Test', RK_ID__c='106',PR_FE_Branded__c= true);
        insert a;
        String orgId=a.id;
        
        Contact c = new Contact();
    
        c.firstname = 'Wain';
        c.lastname = 'R';
        c.AccountId = orgId;
        c.email = 'Wain@test.com';
        c.phone = '6884382997';
        c.mailingstreet = '123 Test Ave';
        c.mailingcity = 'Somewhere';
        c.mailingstate = 'MA';
        c.mailingPostalCode = '87945';
        c.mailingcountry = 'US';
        c.User_Num__c='1234567';
        insert(c);
        
        RestRequest req = new RestRequest(); 
        RestResponse res = new RestResponse();
        
        REST_ParticipantHandler.Wrapclass reqst=new REST_ParticipantHandler.Wrapclass();
        reqst.status= 'Success';
        reqst.responseBody ='{"state":"CA","riskTolerance":"Typical for your age","retirementAge":"70",alPortfolio":"65627.50","phoneContactPreference":"SEC Required Only","email":"","homePhone":"5555555555","title":"","mailingAddress":"13630 SE 231ST ST","userNum":"13328911","lastInitial":"R","city":"Sunnyvale","zip":"94086","foreignResident":"0","workPhone":"4084986934","otherPhone":"5555555555",","isFeBrand":"1","primaryAccountPlan":"Voluntary Investment Plan (VIP)","convertedFromUserNum":"","convertedToUserNum":"","countryCode":"US"}';
        String JsonMsg=JSON.serialize(reqst);

        req.requestURI ='/services/apexrest/UpdateParticipant'; 
        req.httpMethod = 'POST';
        req.requestBody = Blob.valueof(JsonMsg);
        RestContext.response= res;
        RestContext.request = req;
        REST_ParticipantHandler.UpdateParticipant();
        
        //System.assertEquals('true', results.success);
        //System.assertEquals(10, results.records.size());
        
      }
}

*****************************************************************
Im getting an error "System.QueryException: unexpected token: 'FROM'" 
**********************************************************************